The Sickness Unto Death by Søren Kierkegaard

The Sickness Unto Death (Danish: Sygdommen til Døden) is a book written by Danish philosopher Søren Kierkegaard in 1849 under the pseudonym Anti-Climacus. A work of Christian existentialism, the book is about Kierkegaard's concept of despair, which he equates with the Christian concept of sin, particularly original sin.